Some 49ers have workout bonuses as high as $500,000 for participating in the team's offseason program. (49ers.com)

The 49ers opened their "voluntary" offseason program on Monday at their Santa Clara practice facility.

Yes, it's called a voluntary program. But it's mandatory if some of the team's top players wish to receive lucrative workout bonuses written into their contracts.

The 49ers have made it a priority for their most-notable veteran players to participate in the offseason program. Millions of dollars are rewarded to players who take part in the team's nine-week offseason program, which explains why the team expects another offseason with attendance near 100 percent. After all, if the veteran starters attend the offseason program, you can be sure that all the younger players fighting for jobs will be there, too.

Wide receiver Michael Crabtree agreed to a contract in 2009 that contained a so-called "diva clause," which requires full participation in all mandatory functions and 90-percent attendance in voluntary work in order to collect millions of dollars in base salary escalators.

Recently, Anthony Davis signed a deal that includes $7.25 million in "de-escalators" over seven years -- and $250,000 this offseason -- if he fails to meet workout and weight requirements.

Linebacker Patrick Willis and defensive lineman Justin Smith have $500,000 workout bonuses in their contracts. Smith is expected to be healthy in May after undergoing surgery to repair a 50-percent torn left triceps tendon shortly after the Super Bowl.

Running back Frank Gore had never participated in the 49ers' offseason program, opting to work out with a trainer in his hometown of Miami, until he signed a deal that includes $400,000 annual workout bonuses.

Defensive lineman Glenn Dorsey, signed to start at nose tackle this season, has a $350,000 workout bonus in his contract. Tight end Vernon Davis and left tackle Joe Staley have bonuses of $200,000 for participating in the offseason program.

Veteran cornerback Nnamdi Asomugha, who took part in the first day of the offseason program, has a $25,000 workout bonus in his deal. Safety Craig Dahl and linebacker Dan Skuta, two other free agent acquisitions, have $50,000 workout bonuses.

During the first two weeks of the NFL's offseason program, only strength and conditioning coaches may be on the field with the players. Head strength and conditioning coach Mark Uyeyama and assistant Kevin Tolbert run this phase of the offseason. Uyeyama and Tolbert lead two groups of players through their structured workouts on a daily basis. Players may choose to work out in the morning or afternoon.

Jim Harbaugh, coordinators Greg Roman, Vic Fangio and Brad Seely, and the position coaches are not allowed to observe the workouts, which are limited to "dead ball" activities. Quarterbacks may throw to uncovered receivers and no helmets are allowed to be worn.